Mac: main thread is blocked on startup for some time

On application start, the main thread (and subsequently the UI) is sometimes blocked for quite some time (approx. 10 seconds or even more).

Beach ball is seen if you try to click anything. The UI unblocks automatically after this time period.

It got worse with os x 10.10, tested both by me and felix.

I'll attach both the spindump I made, and the output of Instruments spin monitor.